1. Field of the Invention
The invention relates generally to the field of marine geophysical surveying. More particularly, the invention relates to methods and systems for adaptive speed control of towed marine streamers.
2. Background Art
Marine geophysical surveying systems include towed electromagnetic sensor streamers. Such sensor streamers are towed by a vessel at a selected depth in a body of water and include a plurality of electric field and/or magnetic field sensors disposed along the length of the streamer. The sensors generate signals related to electromagnetic fields induced in formations below the bottom of the body of water by a transmitter. See, for example U.S. Patent Application No. 2010/0017133.
As well explained in the '133 publication, a significant source of noise in the signals detected by the sensors in an electromagnetic streamer cable is from motion of the cable in the Earth's magnetic field. In marine seismic surveying, seismic streamers are towed at relatively shallow depths in the water, and for practical purposes it is assumed that the streamer velocity is approximately the same as the velocity of the towing vessel. In the case of marine electromagnetic surveying, the sensor streamers are towed at greater depth in the water. There is thus a weaker correlation between the velocity of the streamers in the water and the velocity of the tow vessel. If the actual velocity of the streamers at depth in the water becomes excessive as a result of subsurface water currents, excessive noise may be induced in the measurements made by the electromagnetic sensors.
There exists a need for measurement and control of streamer velocity at the selected towing depth.